Among surface light emitting bodies, an EL device is expected to be used for a flat panel display, and it is furthermore expected to be used as next-generation luminaries for replacing fluorescent lamps or the like.
The structure of an EL device is diversified, that is, from a simple structure in which an organic thin film to be a light emitting layer is merely disposed between two films to a multi-layered structure. As an example of the latter multi-layered structure, those obtained by laminating a positive hole transport layer, a light emitting layer, an electron transport layer, and a negative electrode on a positive electrode formed on a glass substrate can be mentioned. The layer sandwiched between the positive electrode and negative electrode entirely consists of an organic thin film, and the thickness of each organic thin film is extremely thin, that is, only several tens of nm.
The EL device is a laminate of thin films, and based on a difference in refractive index between the materials of each thin film, the total reflection angle of the light between thin films is determined. Under the current circumstances, about 80% of the light generated from the light emitting layer are trapped inside the EL device and cannot be extracted to the outside. Specifically, when the refractive index of a glass substrate is 1.5 and the refractive index of an air layer is 1.0, the critical angle θc is 41.8° and the light with the incident angle lower than the critical angle θc is emitted from the glass substrate to the air layer. However, the light with the incident angle higher than the critical angle θc undergoes total reflection and is trapped inside the glass substrate. For such reasons, it has been desired to achieve the light extracting function for extracting the light trapped inside a glass substrate on a surface of an EL device to the outside of the glass substrate, that is, to improve the light extraction efficiency and luminance in the normal direction.
Further, with regard to an EL device which performs isotropic light emission, it is also desired to have not only improvement in the light extraction efficiency and luminance in the normal direction but also small emission angle dependence of the wavelength of light emitted from an EL device. Specifically, it is desired to have a small difference in emission angle according to the wavelength when emitted light from a light emitting layer passes through a glass substrate to result in light emission from the glass substrate. In other words, the wavelength dependence of the light distribution emitted from a glass substrate is desired to be as small as possible.
To solve the problems described above, a film containing light diffusion fine particles is suggested in Patent Document 1. Further, a film having a convex-concave structure with a pyramid shape is suggested in Patent Document 2.